Directly-modulated semiconductor lasers have been used for providing small-size optical transmitters with low power consumption and capable of operating without requiring any temperature control. At present, semiconductor lasers, which provide direct modulation at operation speeds of up to 10 Gb/s, are available for practical use.
In order to further enhance their operation speeds, the relaxation oscillation frequency, which is a limiting factor hindering an increased operation speed of a laser, is needed to be increased. For higher relaxation oscillation frequency, reduction in the cavity length is essential.
As such semiconductor lasers directed to higher-speed operation, proposed are short cavity DFB lasers depicted in FIG. 9, and short cavity DBR lasers depicted in FIG. 10.
In addition, proposed is integration of a passive grating waveguide at the rear of a DFB laser having a λ/4 phase shift structure so as to increase the output from the front, thereby effectively enhancing the reflectance at the rear.